The use of oligomeric photoinitiators in photopolymerisation has several advantages in comparison with the use of monomeric photoinitiators, such as lower migratability of the photoinitiator and reduced amount of volatile compounds derived from their photodecomposition. Those characteristics are important for the industrial use of the photonitiator because they reduce the risk of contamination of the finished products.
Among the known oligomeric photoinitiators, the alpha-hydroxycarbonyl derivatives of oligomers of alpha-methylstyrene of formula A, wherein n is a number equal or greater than zero, are mostly appreciated in the field.

These photoinitiators are described in U.S. Pat. No. 4,987,159 and they are mainly constituted by mixtures of dimer and trimer isomers. At room temperature these mixtures of dimer and trimer isomers are highly viscous products that usually require pre-heating for easy handling.
As a consequence, solid mixtures of alpha hydroxycarbonyl derivatives of alpha-methylstyrene oligomers in powder form have been developed and are now highly appreciated photoinitators for photopolymerising acrylic systems.
Their composition and synthesis is reported in EP 1389177.
The solid mixtures of alpha hydroxycarbonyl derivatives of alpha-methylstyrene oligomers of EP 1389177 contain about 90-98% of two dimer isomers: 5-[4-(2-hydroxy-2-methyl)-1-oxo-prop-1-yl]-3-[4-(2-hydroxy-2-methyl)-1-oxo-prop-1-yl-phenyl]-2,3-dihydro-1,1,3-trimethyl-1H-indene (dimer isomer 5) and 6-[4-(2-hydroxy-2-methyl)-1-oxo-prop-1-yl]-3-[4-(2-hydroxy-2-methyl)-1-oxo-prop-1-yl-phenyl]-2,3-dihydro-1,1,3-trimethyl-1H-indene (dimer isomer 6).
Dimer isomer 5 is the compound of formula V:

Dimer isomer 6 is the compound of formula VI

The process of EP 1389177 provides these solid mixtures through controlled crystallization of the high viscosity mixtures of alpha-hydroxycarbonyl derivatives of oligomers of alpha-methylstyrene, whose synthesis is described, for instance, in U.S. Pat. No. 4,987,159.
Both dimer isomers are active as photoinitiators, but dimer isomer 5 is more reactive in photopolymerization than dimer isomer 6, as it is also reported in EP 1389177.
One of the advantage of the controlled crystallization of EP 1389177 is that it provides solid mixtures of alpha hydroxycarbonyl derivatives of alpha-methylstyrene oligomers that are enriched in dimer isomer 5.
A process for the preparation of a crystalline mixture of dimer isomer V and VI is also described in EP 1620382. The process uses 1,1,3-trimethyl-1-phenylindan as the starting product and may be adapted for the preparation of the individual dimer isomer V through separation of one of the intermediates.
Both the process of EP 1389177 and the process of EP 1620382 prepare alpha-hydroxycarbonyl derivatives of alpha-methylstyrene dimers by acylation of alpha-methylstyrene oligomers or dimers. Due to the fact that acylation occurs both in the 5- and 6-positions, mixtures of 5- and 6-isomers are always obtained and the preparation of dimer isomer 5 in enriched or isolated form implies the discharge or separate use of the less reactive dimer isomer 6.
As a consequence it would be highly desirable to design a short, efficient and isomer 6-free synthesis of dimer isomer 5 which also dispenses with costly and inconvenient purification steps and provides the product in solid and pure form.
It has now been found that this objective is achieved by a process that comprises the acylation of cumene in the 4-position with an isobutyryl halide, followed by benzylic halogenation and dimerization (cyclization) of the resulting product.